This pattern is used when you guess a cause or reason for something, but you are not entirely certain—you say 'perhaps because'. It often carries a nuance of attributing fault or responsibility to that reason, so it can sound slightly accusatory or complaining. For example, if someone is tired, you might say it's because they stayed up late, but you're not sure.

It attaches to the plain form: for verbs and adjectives, use the plain present or past (e.g., 忙しいせいか, 来なかったせいか). For nouns and な-adjectives, you need の before せいか (e.g., 雨のせいか, 静かなのせいか). The pattern is used in both speech and writing, but be careful: it suggests you think the reason is somehow negative or undesirable. For a neutral or positive cause, use おかげで instead.